c - Difficulties with understanding reallocating using malloc -

i trying increase size of collection structure using malloc instead of realloc. i'm afraid i've made mistakes while assigning variables because i'm getting:

malloc: *** error object 0x7fd46b404ac8: pointer being freed not allocated 

i appreciate if give me helpful hint.

void increasecollectionsize(){   collection.size = (collection.size + 1) * reallocate_modifier;   char **increasedcollection = malloc(sizeof(char *) * collection.size);   char **increasedhead = increasedcollection;   for(int = 0; < collection.numberofwords; i++){     *increasedcollection = *collection.words;     increasedcollection++;     collection.words++;   }   free(collection.words); // i'm getting error here.   collection.words = increasedhead; }  typedef struct collection{   char **words;   size_t numberofwords;   size_t size; } collection; 

cheers!

you free final value of collection.words, pointer end of original memory block (due ++ in loop).

void increasecollectionsize(){     collection.size = (collection.size + 1) * reallocate_modifier;     char **increasedcollection = malloc(sizeof(char *) * collection.size);     char **increasedhead = increasedcollection;     char **originalwords = collection.words; // save pointer     for(int = 0; < collection.numberofwords; i++){         *increasedcollection = *collection.words;         increasedcollection++;         collection.words++;     }     free(originalwords); // ok     collection.words = increasedhead; } 

or, reducing complexity:

void increasecollectionsize() {     collection.size = (collection.size + 1) * reallocate_modifier;     char **increasedcollection = malloc(sizeof(char *) * collection.size);     assert(increasedcollection != null); // explicitly abort() on malloc() error      (size_t = 0; < collection.numberofwords; i++)         increasedcollection[i] = collection.words[i];      free(collection.words);     collection.words = increasedcollection; }